When electrosurgical cutting with an electrode is initiated, the tissue presents a low impedance path to the Radio Frequency (RF) current so that, at a given voltage, a significant amount of RF current (and RF power) may flow through the tissue. When this current heats the tissue from body temperature to greater than about 100° C., the fluid in the tissue starts vaporizing. The impedance begins increasing as the electrode is enveloped by a thin vapor bubble/layer.
Once the vapor envelops the electrode, it interrupts the current and the full voltage of the generator may be applied across the thin vapor layer to create a high electric field in the vapor bubble. This high electric field exerts force on the ions present in the vapor, accelerating them and establishing the current flow across the vapor gap. Impedance is understood to start decreasing (ionization phase) as a plasma develops. As the ions are accelerated, they are understood to collide with the molecules present in the vapor bubble, further ionizing them and leading to spark discharge. As the voltage across the vapor gap is present, it is understood to further accelerate the ions in the plasma, increasing their kinetic energy and thus temperature of the plasma which may eventually lead to avalanche ionization, high energy and arc discharge.
Many RF systems use a monopolar configuration for electrosurgical cutting. Such a device has an active electrode at its tip that is applied to the tissue to be cut. The return electrode is often in the form of a ground pad dispersive electrode that is placed on a patient's body in a different location than the area of surgery. An electrical circuit forms between the active electrode and return electrode through the patient. Since the path of the current through the patient is not precisely defined and is dependent on the local conditions of the tissue, the monopolar configuration is not the best to use in the proximity of sensitive organs or structures.
An electrosurgical device in a bipolar configuration, with the return electrode next to the active electrode, is a much safer device in such circumstances. An electrical circuit forms between the two electrodes, removing the need for current to flow through a patient's body to the ground pad as the monopolar configuration requires. One shortcoming of the bipolar configuration, however, is its need to establish two points of contact with the tissue to initiate cutting. The two-point contact is dependent on the angle of the handpiece with respect to the tissue surface. This dependence on angle may make it necessary to tilt the handpiece to establish a good contact and ignite the plasma.
One bipolar electrosurgical device addressed this problem with a spring-loaded return electrode to provide a self-compensating function. Coagulated blood or accumulated tissue may impede the proper function of such a device, however. Another variation involves coblation, which uses saline as a return electrode. This approach, however, requires the electrode to be submerged in saline for the duration of the task.